Damage Prevention in Transformer Bushings Subjected to High-velocity Impact

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Ballistic impact, Pressurized brittle cylinders, Experimental modeling, Transformer bushings, Probability

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Daniel Felix Ritchie School of Engineering and Computer Science, Mechanical and Materials Engineering


This paper contributes further to the discussion recently initiated in Ref. [1] regarding the protection of pressurized porcelain bushings against high-velocity impacts on high-voltage (HV) transformers by means of an elastomeric coating. Valuable observations and conclusions were made in our previous work concerning the impact behavior of the bushings and their protection from a unique combination of ballistic air gun tests performed on borosilicate glass cylinders, representing the bushings, and from flat glass plates tested via drop tower. Clearly, the conclusions from our previous work, critical to many utilities world-wide, needed to be independently verified by full-scale ballistic testing performed on the actual bushings under rifle bullet impact. In addition, to complete the impact analysis, the coated borosilicate cylinders were subjected to impact at energies higher than an air gun to demonstrate the energy effect on cylinder failure. Also, the cascading failures of the cylinders was investigated in this work, for the first time, to show what a single unprotected bushing failure could do to surrounding infrastructure. It was shown that the cascading failure of pressurized unprotected bushings could be a real and dangerous threat in a substation. It was also found that the linear extrapolations of fragmentation behavior with respect to energy in [1] is misleading. Most importantly, the unique but slightly speculative predictions of coating protections on the bushings were significantly overestimated, which was clearly demonstrated in this work. The bushings could be protected by an elastomeric coating against rifle bullet impacts with a coating thickness of a few mm instead of 60 mm as previously reported.

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